European Space Weather Week and exploring Zagreb

Harriet Turner –

This trip contained several firsts for me – first flight, first international conference and first in-person conference presentation. The 18th annual European Space Weather Week was held in Zagreb, Croatia from 24th to 28th October 2022, with delegates from Europe, the US and Australia present. The week was full of interesting talks and lively socials, culminating with the Reading lot (plus a postdoc from Imperial) completing the “secret social” lunar themed escape room in the second fastest time of the week.

It always feels more official when your name is on a lanyard!

The week started with the standard conference registration followed by some tutorials and a live space weather forecast. Space weather refers to the changing plasma conditions in near-Earth space, which can pose a threat to modern life. It can lead to communication failures, damage to satellites, blackouts, and harm the health of humans in space. For this reason, it is important to forecast space weather so that these impacts can be mitigated against. The afternoon of the first day consisted of two parallel sessions and a poster session, with a reception buffet in the evening. The parallel sessions ran throughout the week, covering a wide range of topics from ways to improve our space weather forecasting capabilities to measuring and modelling geoelectric fields.

Tuesday was filled with more parallel sessions containing a wide range of talks, including my first in-person conference presentation of my PhD. I was rather nervous to present my work in front of a (quite large) room full of experts in the field, however I think it was well received and I had some interesting questions.

I study the solar wind, which is a constant stream of charged particles that flows off the Sun and is an important component of space weather. I have been using data assimilation (DA) to forecast the solar wind, which combines model output and observations to form an optimum estimation of reality. For DA to work in an operational context, it needs to work with real time data. This often contains more data gaps and erroneous observations when compared with the cleaned-up science level data, which has been used for previous analysis of solar wind DA. To cut a long story short, my work has shown that the real time data does not significantly worsen the forecasts, meaning that DA could be used for operational solar wind forecasting. Which is what we wanted to hear! I celebrated the presentation being over with a big pizza, followed by the conference music night hosted in a local bar. Turns out there are some talented musicians in the space weather community!

On the stage presenting the slide on the data assimilation scheme I have been analysing.

The rest of the week went by in a blur of parallel and poster sessions, with the conference dinner on the Thursday evening and everything wrapping up on Friday lunchtime. With flights back to the UK not until Monday evening, we had plenty of time to explore what Zagreb had to offer. Saturday was spent exploring the Mushroom Museum (spoiler alert, it was full of mushrooms) and the Museum of Broken Relationships. The latter of the two was filled with donated items that were special in some way or another and symbolised the end of a meaningful relationship. There were certainly some quirky exhibits, but a good attraction for sure.

There was not mushroom for anything else… (I’ll show myself out).

We filled Sunday with a tram ride to the north of Zagreb to the Sljeme cable car. The cable car took us from 267m up to the mountain summit at 1030m, which, for context, is 55m lower than Snowdon. One thing that will remain with me is just how foggy it was in Zagreb, so rising out of the fog in the cable car provided some great views. We could see the cloud hanging low in the valley and it was glorious sunshine at the top. The mountains were covered in trees that were turning into their autumn colours, which certainly was a beautiful sight.

With most of Monday to spare, we explored another museum. This time it was the Museum of Illusions, which was a lot of fun. There were a lot of interactive exhibits, including ones where you can make a kaleidoscope of your own face and play poker with 7 versions of yourself. It led to some truly horrifying photos.

View from the cable car over the mountains.

Overall, it was a tiring yet productive and enjoyable trip. I enjoyed networking with many scientists in my field, many of whom I had only seen as a name on a paper or on Twitter. It was great to see how work in the field is advancing and I look forward to being a part of that in the future.

Finally, a tip if you are visiting Croatia, try Čoksa salted peanut chocolate, it’s great. The forest fruits flavour is also great, but the banana has received mixed reviews!

Diagnosing solar wind forecast errors

Harriet Turner –

The solar wind is a continual outflow of charged particles that comes off the Sun, ranging in speed from 250 to 800 km s-1. During the first six months of my PhD, I have been investigating the errors in a type of solar wind forecast that uses spacecraft observations, known as corotation forecasts. This was the topic of my first paper, where I focussed on extracting the forecast error that occurs due to a separation in the spacecraft latitude. I found that up to a latitudinal separation of 6 degrees, the error contribution was approximately constant. Above 6 degrees, the error contribution increases as the latitudinal separation increases. In this blog post I will explain the importance of forecasting the solar wind and the principle behind corotation forecasts. I will also explain how this work has wider implications for future space missions and solar wind forecasting.

The term “space weather” refers to the changing conditions in near-Earth space. Extreme space weather events can cause several effects on Earth, such as damaging power grids, disrupting communications, knocking out satellites and harming the health of humans in space or on high-altitude flights (Cannon, 2013). These effects are summarised in Figure 1. It is therefore important to accurately forecast space weather to help mitigate against these effects. Knowledge of the background solar wind is an important aspect of space weather forecasting as it modulates the severity of extreme events. This can be achieved through three-dimensional computer simulations or through more simple methods, such as corotation forecasts as discussed below.

Figure 1. Cosmic rays, solar energetic particles, solar flare radiation, coronal mass ejections and energetic radiation belt particles cause space weather. Subsequently, this produces a number of effects on Earth. Source: ESA.

Solar wind flow is mostly radial away from the Sun, however the fast/slow structure of the solar wind rotates round with the Sun. If you were looking down on the ecliptic plane (where the planets lie, at roughly the Sun’s equator), then you would see a spiral shape of fast and slow solar wind, as in Figure 2. This makes a full rotation in approximately 27 days. As this rotates around, it allows us to use observations on this plane as a forecast for a point further on in that rotation, assuming a steady-state solar wind (i.e., the solar wind does not evolve in time). For example, in Figure 2, an observation from the spacecraft represented by the red square could be used as a forecast at Earth (blue circle), some time later. This time depends on the longitudinal separation between the two points, as this determines the time it takes for the Sun to rotate through that angle.

Figure 2. The spiral structure of the solar wind, which rotates anticlockwise. Here, STA and STB are the STEREO-A and STEREO-B spacecraft respectively. The solar wind shown here is the radial component. Source: HUXt model (Owens et al, 2020).

In my recent paper I have been investigating how the corotation forecast error varies with the latitudinal separation of the observation and forecast points.  Latitudinal separation varies throughout the year, and it was theorised that it should have a significant impact on the accuracy of corotation forecasts. I used the two spacecraft from the STEREO mission, which are on the same plane as Earth, and a dataset for near-Earth. This allowed for six different configurations to compute corotation forecasts, with a maximum latitudinal separation of 14 degrees. I analysed the 18-month period from August 2009 to February 2011 to help eliminate other affecting variables. Figure 3 shows the relationship between forecast error and latitudinal separation. Up to approximately 6 degrees, there is no significant relationship between error and latitudinal separation. Above this, however, the error increases approximately linearly with the latitudinal separation.

Figure 3. Variation of forecast error with the latitudinal separation between the spacecraft making the observation and the forecast location. Error bars span one standard error on the mean.

This work has implications for the future Lagrange space weather monitoring mission, due for launch in 2027. The Lagrange spacecraft will be stationed in a gravitational null, 60degrees in longitude behind Earth on the ecliptic plane. Gravitational nulls occur when the gravitational fields between two or more massive bodies balance out. There are five of these nulls, called the Lagrange points, and locating a spacecraft at one reduces the amount of fuel needed to stay in position. The goal of the Lagrange mission is to provide a side-on view of the Sun-Earth line, but it also presents an opportunity for consistent corotation forecasts to be generated at Earth. However, the Lagrange spacecraft will oscillate in latitude compared to Earth, up to a maximum of about 5 degrees. My results indicate that the error contribution from latitudinal separation would be approximately constant.

The next steps are to use this information to help improve the performance of solar wind data assimilation. Data assimilation (DA) has led to large improvements in terrestrial weather forecasting and is beginning to be used in space weather forecasting. DA combines observations and model output to find an optimum estimation of reality. The latitudinal information found here can be used to inform the DA scheme how to better handle the observations and to, hopefully, produce an improved solar wind representation.

The work I have discussed here has been accepted into the AGU Space Weather journal and is available at


Cannon, P.S., 2013. Extreme space weather – A report published by the UK royal academy of engineering. Space Weather, 11(4), 138-139.

ESA, 2018. 

Owens, M.J., Lang, M.S., Barnard, L., Riley, P., Ben-Nun, M., Scott, C.J., Lockwood, M., Reiss, M.A., Arge, C.N. & Gonzi, S., 2020. A Computationally Efficient, Time-Dependent Model of the Solar Wind for use as a Surrogate to Three-Dimensional Numerical Magnetohydrodynamic Simulations. Solar Physics, 295(3),